Chahar Shanbeh Soori is one of the Persian holidays with the least amount of documented history. I personally never remember anyone teaching me the story behind this largely celebrated festival, why it is on a Wednesday, or why it is actually celebrated on Tuesday night? Wikipedia has a two paragraph article that says it has been celebrated since 1700 BCE, and other sources describe some of the old beliefs and rituals including bonfires, going door to door and asking for candy, making statues of those who passed and reunion with their spirit, amazingly similar to Halloween as it was originally celebrated by the Celtics. Although some believe it is a Zoroastrian tradition, if there is one article you must read, it would be this one claiming Chahar Shanheh Soori is even more ancient than Zoroaster.